1. Field of the Invention
The present invention relates to a printing apparatus and a method of acquiring correction value. Specifically, the invention relates to a technique to acquire a correction value to correct an error in conveying a printing medium used in an inkjet printing apparatus.
2. Description of the Related Art
An inkjet printing apparatus has a print head that has a fine-nozzle array, and ink is ejected from each nozzle in accordance with printing data. The ejected ink forms dots on the printing medium to form an image. Accordingly, to form a high-quality image, it is important that the dots should be formed on the printing medium at intended positions. The displacement of the dot-formation position has to be avoided as much as possible. Some of the various causes of such displacement deviation are: difference in shape amongst the nozzles of the print head; noise factors, such as the vibrations of the apparatus that occur while the printing is being carried out; and the distance between the printing medium and the print head. The inventors of the present invention have discovered that one of the significant causes for such displacement deviation of the dot-formation position is the lack of accuracy in conveying the printing medium. One of the commonly used conveying units for the printing medium is a roller (a conveying roller). Conveying the printing medium by a desired distance can be achieved by rotation of the conveying roller by a designated angle with the conveying roller being pressed onto the printing medium. Here, the accuracy in the conveying of the printing medium depends, to a significant extent, on the eccentricity of the conveying roller.
FIGS. 33, 34A and 34B, and 35 illustrate cross sectional shapes of various conveying rollers. The conveying roller of FIG. 33 has a perfectly-circular cross-sectional shape, and has its central axis aligned exactly with its rotational axis. The conveying roller of FIGS. 34A and 34B has a cross-sectional shape that is not a perfect circle. The conveying roller of FIG. 35 has its rotational axis offset from its central axis.
Assume such a case as shown in FIG. 33, or, to be more specific, a case where the cross-sectional shape of the conveying roller is a perfect circle and where the central axis of the conveying roller is aligned exactly with its rotational axis. In addition, further assume that the rotational angle to convey the printing medium is uniform. Then, every rotation of the conveying roller by an angle R constantly gives a particular length (L0) in the circumferential directions (length of arc). Accordingly, every position within the conveying roller always gives a uniform amount of conveying the printing medium that is conveyed while being in contact with the conveying roller.
Contrasting outcomes are obtained by a conveying roller with an ellipsoidal cross-sectional shape as ones shown in FIGS. 34A and 34B. Such a conveying roller gives different amount of conveying even when the conveying roller rotates by the same angle R. This difference in the amount of conveying depends on the rotational position of the conveying roller. To be more specific, for the rotational position shown in FIG. 34A, the printing medium is conveyed by an amount L1 while for another rotational position shown in FIG. 34B, the printing medium is conveyed by an amount L2. Here, the lengths L0, L1, and L2 has such a relationship as L1>L0>L2. That is to say, a periodical variation in amount of conveying the printing medium occurs, and the variation depends on the period of the conveying roller.
Alternatively, as in the case of FIG. 35, the offsetting of the rotational axis of the conveying roller from the central axis O that is intended to be the rotational axis may sometimes cause the amount of conveying the printing medium to vary periodically in response to the period of the conveying roller. To be more specific, assume cases where the rotational axis is offset from the central axis O and is positioned at either the point A or the point B shown in FIG. 35. In these cases, the same rotational angle α produces different amounts of conveying. Such difference in conveying amount results in a periodical variation in the conveying of the printing medium. Here, the variation depends on the period of the conveying roller.
The eccentricity of the roller, which has been mentioned above, includes these above-described states. Specifically, included are a state where the roller has a cross-sectional shape that is not a perfect circle, and a state where the conveying roller has its rotational axis offset from its central axis. In the case of an ideal accuracy being achieved in conveying, the image should be printed in such a way as shown in the schematic diagram of FIG. 36A. With the above-mentioned eccentricity, however, the printed image will be an uneven image with stripes that appear periodically in the conveying direction as shown in FIG. 36B while the period is the same as the amount of conveying corresponding to a full rotation of the conveying roller.
The amount of eccentricity for the conveying roller is usually controlled so as to stay within a certain range. The stricter the standard for the amount of eccentricity is, the lower the yielding of the conveying roller becomes. Accordingly, the printing apparatus thus produced becomes more expensive. For this reason, an excessively strict standard for the amount of eccentricity is not preferable.
To address the above-mentioned problem, various measures have been proposed. Different correction values for the conveying errors are set for different phases of the conveying roller so that even an eccentric conveying roller can achieve a steady amount of conveying as similar to the case of a conveying roller with a perfectly-circular cross-sectional shape and with its rotation axis being aligned exactly with its central axis (Japanese Patent Laid-Open No. 2006-240055 and Japanese Patent Laid-Open No. 2006-272957). To be more specific, correction to reduce the amplitude of the fluctuation in amount of conveying with a period equivalent to the circumferential length of the conveying roller can be done by applying a periodic function with the same period and reversed polarity.
Assume that the conveying roller is manufactured within a predetermined design tolerance. Even in this case, the conveying error that derives from such factors as the amount of eccentricity and the state of eccentricity may sometimes differ between a position and another position in the longitudinal direction of the roller. A roller, which is used in a large-scale inkjet printing apparatus that can print on an A3-sized (297 mm×420 mm) or larger printing medium P, tend to have such a difference that is more prominent than those used in other types of apparatus. Thus, a correction value acquired for correcting an eccentricity-derived conveying error as to a predetermined position of the conveying roller is not always suitable for another position in the longitudinal direction of the conveying roller.